Modern high rise buildings very often do not provide ample means for escaping a fire, earthquake or other emergency affecting the upper floors. Stairwells, elevators and exits may be blocked, engulfed in flames or malfunctioning. Traditional skeletal stairway fire escapes have been eliminated from most modern high rise buildings. Other known methods of emergency escape, such as extendable ladders and safety nets are of limited effectiveness in most modern buildings. The use of such devices is limited to only a few lower floors.
For buildings that exceed 10 stories, airborne rescue by helicopter is often the only available means of escape from the higher floors. However, helicopter rescues are not only expensive, they are very often quite dangerous. The roof of the building may be incapable of sustaining the weight of the helicopter and many logistical problems may accompany the landing of the craft and the rescuing of persons from the building. The number of persons that can be accommodated in a single helicopter is limited and multiple helicopters can interfere with one another and may even risk the danger of colliding in an emergency situation.
As indicated, elevator escape may be unavailable if the elevator is blocked or malfunctioning. Moreover, each elevator is likely to have only a limited amount of space. Retrofitting buildings to include multiple elevators would likely be unduly expensive and architecturally suspect.
At least one city's fire department has trained fire fighters to rescue individuals by repelling down the sides of the building using ropes and other gear previously used in mountain climbing. This technique presents its own difficulties and dangers and obviously is quite limited in the number of persons that can be quickly and safely rescued.
The problems of controlled descent are not limited to high rise buildings. For example, emergency and military personnel are often required to disembark from a helicopter while the craft remains hovering above the earth. Subsequently, these or other persons may have to be hoisted back into the helicopter. Typically, each individual travels along a metallic cable which extends from the craft to the earth. Unfortunately, this cable can effectively act as a grounding device. During stormy or inclement weather, lightning or an electrical discharge can be instantaneously conducted along the cable, thereby creating a severe risk of injury to personnel.